Our invention is adapted to be used in a multiple ratio planetary transmission situated in a vehicle driveline having an internal combustion engine with a throttle control and a hydrokinetic torque converter situated between the engine and input elements of the gearing.
The gearing comprises two simple planetary gear units arranged in a manner similar to the gearing arrangement of the well known Simpson gear set. Located between the turbine of the torque converter and the input elements of the Simpson gearing is a third simple planetary gear unit with a friction clutch adapted to connect two elements of the third gear unit together for rotation in unison. A friction brake also is used for anchoring a reaction element of the third planetary gear unit. An overrunning coupling establishes one-way torque flow between two elements of the gearing. The brake is arranged in series relationship with respect to the clutch.
A second overrunning coupling in a gear unit of the Simpson gearing is used for the purpose of establishing a non-synchronous ratio shift. Forward engagement is achieved by engaging a forward clutch on a shift from neutral to the drive state. Similarly, a separate reverse engagement clutch is used to establish a torque flow path for reverse. In each instance, turbine speed is used as a feedback signal to initiate the start of the forward or reverse engagement.
Ratio changes between the first ratio and the second ratio on an upshift, as well as a downshift from the second ratio to the first ratio, are achieved in our improved transmission by controlling the engagement of an overrunning clutch. The overrunning clutch is arranged in series relationship with respect to a friction brake as a reaction torque flow path for the friction brake associated with the intermediate ratio is established and disestablished. The braking of the friction brake is accomplished with a closed loop control so that harshness is avoided as the overrunning elements of the reaction torque flow path engage. This is in contrast to prior art arrangements, such as that shown in U.S. Pat. No. 5,157,608, where a non-synchronous shift using overrunning couplings is achieved without the cushioning effect made available by the present invention as the associated friction brake is actuated.
Our invention includes a controller for a torque converter clutch assemblies that has a single converter feed passage and a single converter flow return passage. Such converter clutch assemblies are distinguishable from converters of the kind shown, for example, in U.S. Pat. No. 5,305,663 where a converter bypass clutch feed passage acts in cooperation with two other feed passages, one acting as a flow return and the other acting as a flow delivery to the torus circuit of the converter. In the case of the converter shown in the '663 patent, continuous flow is achieved through the converter regardless of whether the clutch is engaged or released.
Portions of the clutch control strategy of the present invention are common to the teachings of U.S. Pat. No. 5,029,087, issued to Ronald T. Cowan, Roger L. Huffmaster and Pramod K. Jain. As in the case of the converter control of the '087 patent, our present invention includes a controller for continuously monitoring the actual converter slip and comparing that actual value to a desired value. Any error that is detected by the controller will result in calculation of a new target slip. During the engagement time of the converter clutch, the error will continuously change and the magnitude of that error will be continuously monitored. In each instance, a new target slip is calculated. This process continues until the actual slip approaches the target value.
This strategy has been adapted to the so-called two pass converter system described above. In the prior art teachings discussed here, the converter is a three pass system that accommodates continuous flow through the converter regardless of the behavior of the clutch. Notwithstanding the fact that the converter has only two flow paths, one toward the converter and the other from the converter, the converter oil flow is capable of maintaining sufficient heat dissipation because of an improved converter flow arrangement in the converter circuit with a converter clutch modulator valve that provides the converter flow. The converter flow is divided into two components, one part of which is directed to a thermostat bypass valve into a lubrication system as the other flow component enters the transmission cooler.
The controller for the converter uses features that are common to the electronically controlled bypass clutch strategy of U.S. Pat. No. 5,303,616 where the percentage of shift completion is used as an input parameter for controlling the engagement of a bypass clutch for a converter, particularly, during ratio changes.
The converter control valve system with which the control strategy is used is capable of accurately adjusting the pressure differential across a converter clutch piston in the converter torus cavity. This involves the use of a single converter pressure modulator solenoid valve, which directly controls the torus cavity pressure. In contrast, two pressure modulators are required in prior art designs in which the release side of the converter clutch piston is controlled as well as the torus cavity pressure. This feature simplifies the converter control valve system of the present invention.
The valve system of the present invention also uses a simplified accumulator valve to create a controlled back pressure on the release side of the converter clutch piston thereby simplifying the control of pressure differential across the converter clutch piston as converter clutch torque capacity is regulated.
The converter clutch control makes it feasible to operate the converter clutch in each ratio using closed loop feedback control. Inertia torque changes during shifts are moderated in this way, thereby reducing shift harshness. The ability to apply the converter clutch in all gear ratios contributes to improved fuel economy.
Improved performance also is achieved by reducing hydrokinetic power loss during acceleration and deceleration.